Stress Corrosion Cracking (SCC) is a type of cracking that occurs only when three factors are present, setting it apart as unique to corrosion. These factors are, tensile stress, environment and material where each material will have specific and generally unique environmental conditions that are required. A microstructural feature that has been shown to impact both the sensitivity to general corrosion and resistance to SCC is the distribution of chromium carbide precipitates. A proposed mechanism for this SCC resistance is crack tip blunting from inter-granular carbides (Bruemmer, Charlot and Henager, 1988). Figure 1 shows one such carbide. The goal of this project to; develop an understanding of the distribution of these carbides in Inconel Alloys, and elucidate the relationship between microstructure and SCC sensitivity. The focus will be on the application of Electron Microscopy based techniques to quantitatively characterize the type, shape and distribution of the carbides with an understanding of local strain. The primary techniques involved will be HR-EBSED for strain, STEM based TKD and EDX for analysis of spatial distribution and type of precipitate.

Stress relaxation cracking (SRC) is a degradation mechanism which occurs in stainless steels and nickel alloys between 550° C and 750°C operation temperature. SRC is a failure mode which can occur during post-weld heat treatment, within 1 to 2 years of service. Cracked regions are located in the heat treatment affected zone (HAZ) or in cold deformed areas subject to long term annealing. An intergranular crack is developed due to high temperature relaxation of internal stress causes local deformation within the microstructure, i.e. a creep failure mechanism.

The project will decouple the impact of microstructural history, chemical composition, temperature and loading conditions using bespoke mechanical testing and characterisation. This will enable us to explore the fundamental mechanisms of creep, stress relaxation, and the role of chemistry in the formation and propagation of microstructurally sensitive cracks during service.

Dwell fatigue was one of the major causes for crack initiation on aeroengine discs, which lead to the reduction of service life. Cold dwell fatigue (CDF) was a complex deformation process that can take place at room temperature. This project is aiming to understand the role of microstructure on damage accumulation in Ti-624x series, with controlled microstructures, and understand how they perform in complex cyclic loading regimes. Major characterisation techniques will be high spatial resolution digital image correlation and high resolution electron backscatter diffraction. Experimental results combined with crystal plasticity modelling will be used to understand the strain partitioning in Ti-624x alloys.

Description: Electron backscatter diffraction (EBSD) is a well-established technique used to probe samples with scanning electron microscopy, where an electron beam is fired at a crystalline material and diffracted from crystal planes to form a Kikuchi pattern. In conventional EBSD, images are processed to extract crystal orientation and maps are formed with systematic mapping of a sample surface.

In this project, I will generate new analysis approaches to improve the precision of the EBSD data obtained and wealth of information probed. I will develop these algorithms using dynamical simulations and use them to probe unknown phases, measure orientation with higher precision and understand deformation in engineering materials.

Re-projection of a dynamical simulation of a ferrite EBSD pattern

Tianhong Gu - EBSD studies of the evolution of microstructure and damage during the thermal cycling of Pb-free solder materials

Title: EBSD studies of the evolution of microstructure and damage during the thermal cycling of Pb-free solder materials

Investigator: Tianhong Gu

Supervisors: Dr Ben Britton / Dr Christopher Gourlay

Funding: Self-funded

Description: When electronics fail in service the cause is often thermomechanical failure of the solder joints. The major failure is cracking occurred in the bulk near solder / substrate interface, as shown in Figure 1. Pb-free joints usually contain more than 95% tin phase, which has highly anisotropic thermophysical properties. Thus, tin is particularly sensitive to thermomechanical fatigue caused by cycles of heating and cooling. There is a strong industrial demand to understand the mechanisms leading to thermomechanical fatigue failure, to be able to predict the joint microstructure that is most resistant to thermal cycling, and to develop ways of generating the optimum microstructure through alloy design and processing.Therefore, this project will be focused on investigating the role of stress distribution within solder joint generated from thermal expansion mismatch between solder ball and substrate, and microstructure of solder ball on failure mechanisms of solder joints during thermal cycling.

Jim Hickey – H2S Corrosion of High Strength Steels

A Project Title – H2S Corrosion of High Strength Steels

Investigator: Jim Hickey

Supervisors: Dr. Ben Britton, Prof. Mary Ryan, Dr. David Payne

Duration: Sep 2015 – March 2019

Funding: EPSRC & Shell UK

Description: In the presence of aqueous H2S and stress, high strength steels (HSSs) (those with yield strengths > 700 MPa) embrittle and can fail catastrophically. This phenomenon is termed sulphide stress cracking (SSC). Understanding this is crucial since H2S containing oil and gas wells (sour gas wells) are now routinely exploited. HSSs are desirable, if not necessary, for some of the infrastructure of oil wells with extreme environments. The aim of the project is to simulate conditions found in down-well environments and characterise the effect of the environment and load on a series of HSSs to guide mechanistic understanding of the effect of H2S on HSSs.

Figure 1 – Inverse Pole Figure Map (Z – perpendicular to sample surface) of a sample of interstitial free steel that has been rolled then annealed at 700 ⁰C for 48 hours. The map is of the rolled surface with the rolling direction depicted.

William White - Discrete dislocation dynamics coupled with discrete solute diffusion to model the effect of hydrogen in steel

Discrete dislocation dynamics coupled with discrete solute diffusion to model the effect of hydrogen in steel

Supervised by Daniel Balint and T. Ben Britton

Funded by ICO CDT Nuclear Engineering and AWE

Abstract

Plastic deformation in metals is due to the motion of mobile dislocations subject to shear stresses in excess of a critical value. At particular strain rates and temperatures mobile solutes form atmospheres at the base of dislocations pinning dislocation motion. This results in an inverse strain response, jerky plastic flow and decreased ductility. In the worst case, ductility is reduced significantly and brittle fracture may occur. We attempt to understand the interaction and effects of mobile dislocations and solutes in metals using discrete dislocation dynamics coupled with an appropriate discrete solute diffusion model. In the first instance our model will be developed to describe carbon solutes in saturated iron. A generalized model will be developed so as to give insight to the deleterious effects of hydrogen in industrial steels.

The shear stress field generated by three edge dislocations in an f.c.c. iron plate modelled using discrete dislocation dynamics. The shear modulus is 80 GPa; Poisson’s ratio is 0.3; the magnitude of the dislocation’s Burgers vector is 0.25 nm.

Metals are widely used for load-bearing applications in complex environments. Their properties are dependent on the underlying behaviour of the material microstructure, which is naturally anisotropic due to the discreet and crystallographic nature of slip and anisotropic elastic properties. This project focuses on developing efficient methods of modelling the evolution of crystallographic texture in two-phase alloys using efficient crystal plasticity based upon the fast Fourier transform. Working with Rolls-Royce plc, materials will be characterised using HR-EBSD to determine textures which will further stimulate the computational work.